Why Bridges Collapse: Engineering Lessons From Famous Failures

Every major bridge failure is, in a sense, a full-scale engineering experiment — one that reveals the limits of design assumptions, inspection regimes, and our understanding of how structures behave over time under real-world loads.

2026-05-16 · By the A2Z News editorial team

The Forces Acting on a Bridge

Before examining failures, it helps to understand what a bridge is constantly fighting against. Every bridge must carry two categories of load. Dead load is the weight of the structure itself — the steel, concrete, and asphalt that sit there permanently. Live load is everything that moves across it: vehicles, pedestrians, trains. On top of these, bridges must resist dynamic forces: wind, seismic shaking, thermal expansion and contraction, and the vibrations induced by traffic passing over joints and imperfections.

Structural engineers design bridges with a safety factor — building in considerably more strength than the expected maximum load demands. A bridge designed to carry 10,000 tons might be built to withstand 30,000 tons before failure. But that safety factor can erode invisibly over decades through corrosion, fatigue cracking, unexpected load growth, and deferred maintenance. When the margin disappears, a routine event can trigger collapse.

Tacoma Narrows (1940): Aeroelastic Flutter

The Tacoma Narrows Bridge in Washington State opened in July 1940 and collapsed into Puget Sound just four months later on November 7, 1940, in winds of only 40 miles per hour — a gentle breeze by bridge-engineering standards. The collapse was captured on film and remains the most studied structural failure in history.

The bridge's designer had used a flat, solid deck plate instead of a truss design, making the bridge exceptionally narrow and aesthetically sleek. This shape, however, was aerodynamically unstable. As wind struck the bridge's roadway, it created alternating vortices above and below the deck in a phenomenon engineers now call vortex shedding. When the frequency of those vortices matched the bridge's natural oscillation frequency, the structure entered resonant vibration — each oscillation was reinforced by the next, building in amplitude until the deck twisted violently and tore apart.

The lesson was profound. Wind tunnel testing of bridge decks became standard practice. Subsequent suspension bridge designs adopted open-truss roadways that let wind pass through rather than build pressure, and computational aerodynamic modeling is now required for any long-span bridge. The Tacoma Narrows disaster effectively created the modern field of bridge aerodynamics.

Silver Bridge (1967): Stress Corrosion and Inspection Gaps

On December 15, 1967, the Silver Bridge connecting Point Pleasant, West Virginia, to Gallipolis, Ohio, collapsed during the evening rush hour, killing 46 people. Investigators traced the failure to a single eyebar — a flat steel link in the suspension chain — that had developed a 0.1-inch stress corrosion crack in a location where it could not be visually inspected without disassembly.

The bridge was an eyebar-chain suspension design used at the time it was built (1928) but made it structurally statically determinate — meaning there was no redundancy. When one element failed, the entire structure had no alternative load path and collapsed in seconds.

The Silver Bridge disaster led directly to the National Bridge Inspection Program, established in 1968, which requires all bridges on the federal highway system to be formally inspected at least every two years. The collapse also drove the principle of structural redundancy: modern bridge designs are intentionally built with multiple load paths so that the failure of a single element does not cause catastrophic collapse.

"The single most important concept in modern bridge safety is redundancy — building in multiple alternative load paths so that no single failure point can bring down the entire structure. The Silver Bridge had none."

I-35W Mississippi River Bridge (2007): Gusset Plate Undersizing

On August 1, 2007, the I-35W bridge in Minneapolis collapsed during the evening rush hour, killing 13 people and injuring 145. The National Transportation Safety Board (NTSB) investigation determined the probable cause: undersized gusset plates — the steel connection plates at the main truss joints — that had been present since the bridge was built in 1967.

The gusset plates were designed with half the thickness they should have had, a calculation error made during the original design process. For nearly 40 years, the plates carried their load within the safety margin. By 2007, however, the bridge was carrying significantly heavier traffic than it was designed for, and on the day of collapse, an unusually large pile of construction equipment and materials was staged on the deck directly above the weakest gusset plates. That additional weight pushed the already-marginal plates past their limit.

The NTSB also found that while fracture-critical elements of the bridge had been flagged in inspections over the years, the gusset plates themselves had never been identified as deficient because visual inspection cannot detect undersizing — it requires review of the original design calculations. The collapse led to new requirements that bridge inspection programs include engineering review of original design documents, not just physical observation.

Fern Hollow Bridge, Pittsburgh (2022): Deferred Maintenance

The January 2022 collapse of the Fern Hollow Bridge in Pittsburgh, while no one was seriously injured, illustrated the ongoing vulnerability of the U.S. bridge inventory. The bridge had been rated "poor" in every inspection since 2011. A 2016 inspection report noted the bridge needed replacement. Yet it remained in service until it buckled and fell on the morning of President Biden's infrastructure bill visit to the city — an irony noted widely.

According to the American Society of Civil Engineers (ASCE), approximately 7.5 percent of U.S. bridges are classified as structurally deficient as of 2021. Structurally deficient does not mean imminent collapse, but it does mean the bridge has elements in poor condition that require monitoring or restriction. The number has improved from about 12 percent a decade ago but remains significant given that the United States has roughly 620,000 bridges.

Fatigue, Corrosion, and the Long War Against Time

Even bridges that are perfectly designed and never inspected inadequately face a relentless physical adversary: time. Steel develops fatigue cracks when subjected to millions of load cycles. Unlike a single overload event that might stretch steel visibly, fatigue cracks grow incrementally — invisibly — until they reach a critical size and propagate rapidly. Highway bridges in heavy-traffic corridors can experience hundreds of millions of load cycles over their design lives.

Corrosion is equally insidious. Road de-icing salt seeps into bridge deck joints, wicks along reinforcing bars in concrete, and dissolves the protective oxide layer on steel members. The result is internal rust that can expand with enough force to crack the surrounding concrete — a process called spalling — while the member's load-bearing cross-section quietly shrinks. Many U.S. bridges built in the mid-20th century were not designed with the assumption that road salt would be used as heavily as it has been.

Modern inspection techniques now go far beyond visual checks. Ground-penetrating radar can identify voids and rebar corrosion inside concrete decks. Acoustic emission sensors detect the sound signatures of crack propagation in real time. Strain gauges embedded in structural members transmit data continuously to monitoring systems. Drone-mounted cameras inspect hard-to-reach areas with photographic resolution. These tools are progressively being integrated into inspection programs as their costs decline.